89Zr-pembrolizumab-PET/CT implements a radiation burden of about 20 millisievert (mSv), and 1.5 mSv per low-dose CT scan. Besides PET imaging, patients will be asked to give in total 12 blood samples (85 mL), which will give minor discomfort. A metastases biopsy will be performed, preferably from an easily accessible lesion to minimize the burden and risk for the patient. Based on a literature review, the risk of tumor biopsies is considered low with a small risk on significant/major complications or death. The risk associated with the 89Zr-pembrolizumab seems minor and although patients do not directly benefit from this study, results of this study will be valuable for our understanding of the tumor immune response and will guide further prospective research.